He ran in last year's elections for a parliamentary seat for the Russian Bloc party, but was not elected.

The killing followed the death of Oleh Kalashnikov, a former member of parliament for Yanukovych's Party of Regions, who was found dead at his home overnight with gunshot wounds.

An Interior Ministry adviser to President Petro Poroshenko linked the deaths, pointing out that both victims had taken part in the Anti-Maidan movement to counter the pro-Western protests in Kyiv that ousted Yanukovych in 2014.

"It seems the shooting of witnesses of the Anti-Maidan affair continues," the adviser, Anton Herashchenko, said in a Facebook post.

Ukrainian President Petro Poroshenko blamed the two deaths on the country's "enemies" who try to "discredit the political choice of the Ukrainian people."

Russian President Vladimir Putin referred specifically to Buzyna's death in a televised call-in show on April 16, describing it as a "political assassination."

"Ukraine is dealing with a whole string of such murders," he said.

U.S. State Department spokeswoman Marie Harf called for "a complete, thorough, and transparent investigation" into the deaths.

Several Yanukovych allies have died in suspicious circumstances over the past three months.

Ukrainian authorities have described most of the deaths as suicides, although officials said some may have been killed or forced to take their lives.

Ukrainian media reported on April 16 that another noted Ukrainian journalist with pro-Russian views, Serhiy Sukhobok, was gunned down three days ago.

Sukhobok came from the Donetsk region in eastern Ukraine, where Ukrainian government forces have been battling pro-Russian separatist militants.

The conflict has killed more than 6,000 people since April 2014.

Yanukovych fled Ukraine in February last year and now lives in Russia.